Machines, such as off-highway vehicle or on-highway vehicle include an engine and a transmission system, such as continuously variable transmission (CVT) coupled to the engine for driving the machine. Nowadays, reduction of emission is given priority while designing the machine. Especially, measures taken for controlling reduction of CO2 emission are also applied on off-highway machines. For reduction of CO2 emission, optimization of the engine may not give desired results; hence increasing efficiency of the transmission system is also an important aspect. Generally, the CVT is used for increasing the efficiency of the transmission system. Although, various types of CVTs are used in machines, the type most often found on larger machines is the parallel path hydrostatic transmission. This type of transmission employs a gear train that receives an input from the engine as well as from a hydraulic motor with continuously variable output. By smoothly varying a speed of the hydraulic motor, final output of the gear train may be continuously varied over a wide range of speeds and/or torque.
The CVT may allow the engine to run at a constant speed. Further, the CVTs may be capable of providing a desired output torque at any speed within its operating range by continuously changing a ratio of the transmission system. However, hydraulic elements used in the hydrostatic CVTs may apply load on the engine, which may cause engine “stalling” or “lugging”. In one example, engine speed variation may be minimized by controlling the ratio of the transmission system. The ratio is controlled based on an error signal determined by comparing an actual speed of the engine to a desired speed of the engine. However, the transmission system needs to be precisely controlled in response to change in load. In another example, a power increase demand may be met by the engine by controlling delivery of fuel in the engine via a fuel governor. The fuel governor is controlled based on the error signal.
U.S. Pat. No. 8,532,889 (the '889 patent) discloses a closed-loop method to control the torque of hydraulic variator. According to '889 patent, torque control mappings are predetermined and become inaccurate due to operating environment variations, machine variations, tolerance changes, and so on. The torque control system further adds a calculated pressure supplement value to each map value before each application of that map value to improve the correlation between the desired or expected and actual output torque of the variator. The effectiveness or accuracy of the disclosed method in '889 patent heavily depends on the determination of the supplement value. The pressure supplement value is actually determined by multiplying the error value by a gain factor, and hence gain factors will directly affect the system performance. The optimized gain factors are operating state dependent, and even may change due to the components wearing. The effectiveness of the closed-loop control heavily depends on the accuracy and effectiveness of the gain selection and optimization. The '889 patent does not discloses further contents regarding gain optimization.